The prevalence of HTLV1 virus antibodies was determined in pregnant women and their neonates in Mashhad, northeast of Iran, as shown in this prospective cross-sectional study. 407 women who were hospitalized for delivery participated in this study. Venous blood sampling of pregnant women and umbilical cord of their neonates was done. The first samples of all women were tested for HTLV1 seropositivity by ELISA test and confirmed by PCR method. Then, the presence of HTLV1 in samples of umbilical cords blood in neonates who were delivered to an HTLV1-positive mother was determined by PCR method. All HTLV1-positive infants were called again at the age of 9–12 months, and PCR test was done using HTLV1-specific primers for them. Of all the participating women, 6 persons were HTLV1 seropositive by ELIZA test which was confirmed by PCR test. HTLV1 antibodies were found in cord blood samples by PCR test in 6 newborns who were born to HTLV1-seropositive women. All the six infants at the age of 9–12 months showed positive PCR results by HTLV1 LTR-specific primers; however, only one of them was PCR positive using HTLV1 TAX-specific primers. The prevalence of HTLV1 antibodies in pregnant women was 1.5%, and the vertical transmission rate to their neonates was 16.6%.